This invention relates to receivers and more specifically, to receiver systems for reception of digitally modulated information.
It is contemplated that digital television services and digital audio services will be digitally modulated when broadcast over satellite and cable television. In particular, a specific digital modulation scheme now in use over satellite is a phase shift keyed (PSK) modulation scheme. In cable systems, it is contemplated that other digital modulation schemes may also be used, such as QAM. These digital modulation schemes are special cases of a class of modulation called pulse amplitude modulation (PAM). The class includes all forms of digital phase and amplitude modulation, such as phase shift keyed modulation (BPSK and QPSK), as well as quadrature amplitude modulation (QAM) which includes a phase modulation component and which is a form of AM-PM modulation, a general class of phase and amplitude modulation. Heretofore, direct broadcast system television services have employed primarily analog modulation schemes for both video and audio information using a variety of low-cost satellite dishes, and low-noise-block (LNB) downconverters. The LNB downconverters provide conversion of the analog signals to frequencies in the L band, as well as amplification of signals for delivery to the indoor unit (IDU), for feed-in to the television unit. In known indoor units, the analog signal from the LNB is fed to a separate L-band downconverter, which in turn down-converts the signal into an intermediate frequency (IF) signal. The IF signal is then demodulated to produce an analog television signal. In a separate technology, digitally-modulated satellite signals are received by a satellite receiving dish and sent to a low-noise downconverter (LNB) for conversion to frequencies in the L band. The indoor unit for digital reception provides a signal through an L-band data downconverter which is down-converted to an IF signal. However, the IF signal, which is a PSK-modulated signal, must be sent to a quadrature downconverter which down-converts the PSK signal to baseband and then outputs two signals in quadrature, one in-phase and one out-of-phase (I and Q). The I and Q signals are then sent to a PSK demodulator which outputs the digital data.
While current satellite receiving dishes, LNBs and certain cable systems can be used for digital television services, the current low-cost direct broadcast L-band downconverters as well as cable converters cannot be used for PAM modulated signals. The L-band downconverters suffer from poor oscillator phase noise due to low synthesizer loop bandwidth in the internal phase locked loop circuits. In addition, there is insufficient dynamic range within any current downconverters since FM video signals operate at fixed power levels, unlike PAM data signals. Finally, there is insufficient image rejection for PAM data reception.
There are available a number of L-band data downconverters and quadrature downconverters for use in specialized low volume applications. However, such downconverters are prohibitively expensive and not well-adapted for use for consumer digital television and digital radio. For example, such downconverters require numerous manual adjustments which are not readily automated. What is therefore needed is a low-cost downconverter or downconverter combination suitable for direct broadcast services for consumer applications.